Basics of Anti-friction BearingsAnd their selection from Manufacturer’s catalogue

Dr. Chandan SharmaDepartment of Mechanical Engineering

Engineering College Ajmer

Functions of a Bearing

• To ensure free rotation of shaft or axle with minimum friction

• To support the shaft and hold it in the correct position

• To take up the forces that acts on the shaft and transmits them to frame

or foundation

Types of Bearing

• Based on direction of force (Radial bearing and thrust bearing)

• Based on friction between shaft and bearing surface (Sliding contactbearing and Rolling contact bearing)

Types of Bearing

• Based on friction between shaft and bearing surface (Sliding contactbearing and Rolling contact bearing)

Applications of Rolling Contact Bearings

Rolling contact bearings are used in following applications:

• Machine tool spindles

• Automobile front and rear axles

• Gear boxes

• Small size electric motors

• Crane hook and hoisting drums

Types of Rolling Contact Bearings

A Rolling contact bearing consist of four parts:

• Inner and outer races

• Rolling element (balls/rollers/needles)

• Cage (for holding rolling element together and spacing them evenly)

Deep groove ball bearings

• Most frequently used bearing

• Point contact between balls and races

Advantages:

High load carrying capacity

Takes load in radial as well as axial directions

Excellent performance in hi-speed applications

Generates less noise due to point contact

Wide range of bore diameter (few mm to 400 mm)

Disadvantages:

Accurate alignment is required

Poor rigidity as compared to Roller bearings

Cylindrical Roller Bearing

• Used for high load carrying capacity

• Relatively short rollers are positioned and guided by cage

Advantages:

High radial load carrying capacity (line contact)

More rigid than DGBB

Low coefficient of friction in high speed applications

Disadvantages:

In general cannot take thrust load

Requires precise alignment

Generates relatively more noise

Needle Bearing (Quill bearing)

• Length to diameter ratio > 4

• Can be used without inner and outer races

• Suitable where limited radial space is available

Advantages:

Compact and light weight

Large load carrying capacity compared with their size

Large load carrying capacity particularly at low speeds

Disadvantages:

Cannot be manufactured with high degree of accuracy

This results in high friction in needle bearings

Coefficient of friction can be up to 4 times than that of cylindrical roller bearing

Angular Contact Bearing

• Line of reaction makes an angle with the axis of bearing

• Often used in pairs to take thrust load from both directions

Advantages:

Can take radial as well as thrust load

More load carrying capacity than DGBB

Disadvantages:

Two bearings are required to take thrust loads

Must be mounted without axial play

Requires initial pre-loading

Self aligning bearings

• Consist of two rows of balls or rollers

• The inner race and balls can freely roll and adjust to the angular misalignment ofthe shaft

Advantages:

Can take high radial as well as thrust load

More suitable where misalignment can arise due to deflection

Disadvantages:

Comparatively costlier

Used in agriculture machinery, railway axle boxes

Can be self aligning ball bearing or spherical roller bearing

Principle of Self aligning bearings

Taper Roller Bearing

• Rolling elements in the form of frustum of cone

• Often used in pairs to balance the thrust component

Advantages:

Can take heavy radial as well as thrust load

These bearings has more rigidity

Can be assembled and disassembled

Disadvantages:

Two bearings are required to take thrust loads

Comparatively costlier

Cannot tolerate misalignment

Thrust ball bearing

• The use of large number of balls results in high thrust load carrying capacity

Disadvantages:

Cannot take radial load

Performance satisfactory at low and medium speeds only

Cannot tolerate misalignment

Does not operate well on horizontal shafts

Continuous pressure is required to hold shaft ring and housing ring

Material of Rolling Contact Bearings

Balls, inner and outer races – high carbon chromium steel (1% C and 1.5 % Cr)

Balls and races are through hardened

Cages are made from stampings of low carbon steel

The rollers are made of case hardened steels

Rollers are case carburized

Balls are through hardened whereas rollers are case hardened

Selection of Rolling Contact Bearings

For low and medium radial loads, ball bearings are used

For heavy loads and large shaft diameters, roller bearings are used

Thrust ball bearings are used for medium thrust loads

For high speed applications, DGBB, ACB, and CRB can be used

For rigidity considerations, roller bearings are preferred over ball bearings

For noise considerations, ball bearings are preferred over roller bearings

Fa/Fr < 0.7 SRDGBB

0.7 < Fa/Fr < 1.5 DRDGBB

Fa/Fr > 1.5 Rolling contact bearings

Factors affecting selection of Rolling Contact Bearings

Static load carrying capacity

Life of a bearing

Dynamic load carrying capacity

Equivalent dynamic load

Load-life relationship

Static load carrying capacity (C0)

It is the load acting on the bearing when the shaft is stationary

It produces permanent deformation in balls and races which increases with increasing load

It is defined as static load which corresponds to a total permanent deformation of

balls and races at the most heavily stressed point of contact, equal to 0.0001 of the

ball diameter

Stirbeck’s equation d is ball diameter, z no. of balls and k

is factor that depends on curvature at

the point of contact

5

zdk C

2

0

Life of a bearing

The life of an individual ball bearing is defined as the number of revolutions (or hours of service at some given constant speed) which the bearings run before the first evidence of fatigue crack in balls or races.

The life of a single bearing is difficult to predict

Hence life is defined in terms of average performance of a group of bearings

L10 life (also called as rated life) is the life which 90% of the bearings will reach or exceed before fatigue failure

Average life or L50 is the life which 50% of the bearings will reach or exceed before fatigue failure

1050 L5 L 6

hmr

10

N L 60 L

Dynamic load carrying capacity (C)

It is based on the fatigue life of the bearing

It is based on the assumption that inner race is rotating and outer race is stationary

It is defined as radial load in radial bearings (or thrust load in thrust bearings) that

can be carried for a minimum life of one million revolutions

The minimum life in this definition is the L10 life, which 90% of the bearings will

reach or exceed before fatigue failure

Equivalent Dynamic load (P)

In actual applications, the force acting on the bearings has two components –radial and axial

It is therefore necessary to convert the two components acting on the bearing into a single hypothetical load fulfilling the conditions applied to the dynamic load carrying capacity

It is defined as constant radial load in radial bearings (or thrust load in thrust

bearings) which if applied to the bearing would give same life as that which the

bearing will attain under actual condition of forces

P is equivalent dynamic load, X and Y are radial and

Thrust load factors and V is race rotation factor

V=1.0 when inner race rotates and 1.2 when outer race rotates

ar F Y FV X P

Load-life relationship

The relationship between the dynamic load carrying capacity, the equivalentdynamic load and the rated life of the bearing is given by:

Where:

L10 is the rated life (in million revolutions)

C is dynamic load carrying capacity

P is equivalent dynamic load

p=3 (for ball bearings) and p=10/3 (for roller bearings)

p

10P

C L

Selection of bearing life

Bearing life for wheel applications (speed of rotation not constant)

Selection of bearing life

Bearing life for industrial applications (speed of rotation constant)

Load factor

Designation of Rolling Contact Bearings

The rolling contact bearing is usually designated by 4 digits:

The last 2 digits indicate the bore diameter in mm (bore diameter divided by 5).For ex. XX15 indicates a bearing of bore diameter 75 mm.

The third digit from right indicates load series of the bearing:

• Extra light series – 0 or 1

• Light series – 2

• Medium series – 3

• Heavy series – 4

The fourth digit (and sometimes 5th digit)

from right specifies the type of rolling

contact bearing.

For example, digit 6 indicates deep groove ball bearing.

Selection of Rolling Contact Bearings from Catalogue

a. Calculate radial and axial loads and shaft diameter (if not given).

b. Select type of bearing for the given application. (or ratio of Fa and Fr)

c. Determine X and Y from the tables.

For that we require Fa / C0 and C0 has to be taken from catalogueHence selection is done by trail and error

d. Calculate equivalent dynamic load P = X Fr + Y Fa

e. Make decision about the expected bearing life and express L10 in million revolution.

f. Calculate dynamic load capacity from load life relationship.

g. Check whether the selected bearing of series has required dynamic capacity or

not. If not, select the bearing of the next series and go back to step (c) and

repeat the procedure until C (calculated) < C(tabulated).

h. Calculate revised life of the bearing for the tabulated value of C using load life

relationship.

Selection of Rolling Contact Bearings from Catalogue

a. Calculate radial and axial loads and shaft diameter (if not given).

b. Select type of bearing for the given application. (or ratio of Fa and Fr)

c. Determine X and Y from the tables.For that we require Fa / C0 and C0 has to be taken from catalogue

Hence selection is done by trail and error

Selection of Rolling Contact Bearings from Catalogue

Selection of X and Y:

Selection of Bearings for Cyclic loads and Speeds

In certain applications, bearings are subjected to cyclic loads and speeds.

For example, (i) radial load of 2500 N at 700 rpm for 25% of time

(ii) radial load of 5000 N at 900 rpm for 50% of time

(iii) radial load of 1000 N at 750 rpm for remaining 25% of time

3

321

3

33

3

22

3

11e

........... N N N

.............. PN PN PN P

Reliability of Bearings

In certain applications, where there is risk to human life, it becomes necessary to selecta bearing having a reliability of more than 90%.

The relationship between life and reliability is given by a statistical curve known as‘Weibull distribution’.

For Weibull distribution:

For L50 = 5 L10, a=6.84 and b = 1.17

under test bearings of no. Total

mr Lcompletedly successful bearings of No. y Reliabilit

b

a

L

e R

Reliability of Bearings

If desired reliability is other than 90%, following formula may be used:

In a system, if there are N bearings, each having a reliability of R, then the completeReliability of system is given by:

b

1

90

e

e

10

R

1log

R

1log

L

L

NS R R

Bearing failure - Causes and remedies

The failure of anti-friction bearings occurs not due to breakage of parts but due todamage of working surfaces of their parts. The principle types of surface wear are asfollows:

Abrasive wear

Corrosive wear

Pitting

Scoring

Abrasive Wear

Causes:

When bearing is made to operate in environment contaminated with dust, foreignparticles or rust

Remedies:

Provision of oil seals

Use of high viscosity oils enables fine particles to pass without scratches

Corrosive Wear

Causes:

When water or moisture enters the bearing

Also caused by corrosive element present in additives in the lubricating oil

Remedies:

Providing complete enclosure free from external contamination

Selecting proper additives

Replacing lubricating oil at regular intervals

Pitting failure

Causes:

When load on the bearing exceeds surface endurance strength

Characterized by pits which continue to grow resulting in complete destruction

Also caused by corrosive element present in additives in the lubricating oil

Remedies:

Improving surface hardness improves surface endurance strength

Scoring failure

Causes:

Excessive surface pressure, high surface speed and inadequate supply of lubricantresults in breakdown of the lubricating film

This results in excessive frictional heat and overheating at the contacting surfaces

Remedies:

Proper selection of parameters such as surface speed, surface pressure and flow oflubricating oil such that resulting temperature at the contacting surfaces is withinpermissible limits

Bearing failure

Lubrication of Rolling Contact Bearing

Objective of lubrication:

Reduce friction between balls/rollers and races

Dissipation of frictional heat

Prevention of corrosion

Protection of bearing from dirt and foreign particles

Oil as lubricant:

More effective in carrying away heat

Feeds more easily into contact areas

More effective in flushing out dirt, corrosive and foreign particles from the bearings

Grease as lubricant:

Simple housing design, less possibility of leakage

Less maintenance cost

Better sealing against rust

Guidelines for selection of lubricant

Temperature less than 100°C, grease is preferred and above 100°C, oil should be used

When bore (in mm) x speed (in rpm) is below 2000000, grease is suitable. For highervalues, oil should be used

Grease is suitable for low and moderate loads while oils are used for heavy dutyapplications

If central lubrication system exist (for lubrication of other parts), oil can be used forlubricating bearings also

Though in majority of applications, grease offers simplest and cheapest mode of lubrication

Mountings of Rolling Contact Bearing

Mountings of Rolling Contact Bearing

For the bearing to function satisfactorily and attain required life, correct method ofmounting and cleanliness should be observed.

Following precautions should be taken:

Mounting should be carried out in dust free and dry environment

Before assembly, the burrs on the shaft and shoulders should be removed

The bearing should not be taken out from its packing before it is assembled

While mounting bearing, direct blows should never be applied to the bearing surfaceotherwise race or cage may get damaged

Large bearings are mounted by heating them up to 80°C to 90°C above the ambienttemperature by induction heating and then shrinking them on the shaft. These shouldnot be heated by direct flame

Thanks for your kind attention